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Cost-effectiveness of Different Revegetation Techniques for Slender Banksia
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For planning revegetation, an understanding of options available to optimize plant establishment, survival, and growth is important, but while there is much published information on revegetation techniques, there is little on costs and cost-effectiveness (Bullock et al. 2011). In this study, I examined the costs associated with two commonly-used revegetation techniques: broadcast direct seeding and manual planting of nursery-grown seedlings. Seeding is generally cheaper, but may take longer to establish cover. Seedling planting, while more expensive, has higher initial establishment success, and may produce canopy cover faster (Whisenant 1999).

I examined the relative success, growth rates, and costs of planting slender Banksia seedlings with various treatments over 18 months, and compared survival rate and establishment costs with those of a broadcast direct seeding experiment. Costs, in 2012 Australian dollars, are for commercial-scale revegetation (Tranen Revegetation Systems, Perth), rather than the actual costs of conducting the small-scale experiments, which would not be truly representative of real-life situations. Weed control costs are not included—they will depend on site weediness and control methods and frequencies used for the particular species and conditions.

Banksia woodlands, widespread in the Perth region of southwest Western Australia, have declined substantially with land clearing, altered fire regimes, dieback, declining rainfall, and groundwater drawdown (Lamont et al. 2007). Slender Banksia (Banksia attenuata ), the most common of 60 local Banksia species (Enright and Lamont 1992a), is prioritized in restoration projects. It has high germination rates, but low survival rates through the first summer (Osborne 1990), and is slow growing (George 1987). Mature slender Banksia develops long tap roots that access groundwater in summer. This species is non-mycorrhizal, and its cluster roots in the upper 10 cm of soil facilitate uptake of near-surface nutrients when soil is moist (Lambers et al. 2010).

Plastic tree guards are commonly used locally to protect seedlings from herbivory and wind, but there are few reports on their effects on Banksias. One recent Perth study (Close et al. 2009) found no significant effects on slender Banksia mortality or growth, but high temperatures recorded inside guards indicated their potential to cause slender Banksia to overheat in warm weather. Fertilizer is often added to promote seedling growth, though high P levels can be detrimental to slender Banksia survival and growth (Fisher et al. 2006). Low-phosphorus native plant fertilizer tablets release nutrients gradually over 12 months. Hydroabsorbent polymers assist plant establishment and growth in drought-prone soils (Woodhouse and Johnson 1991). TerraCottem is a commercially-available product incorporating such polymers, together with fertilizer and root growth activator. Although marketed since 1993, independent research data on its use in revegetation is limited. The present study examined its ability to promote first-year survival and growth of slender Banksia, particularly through providing additional near-surface water in dry periods, after wetting by rainfall.

In this Mediterranean climate, where the flora are adapted to relatively nutrient-poor deep, sandy soils (Lambers et al. 2010), revegetation is usually done in winter, when most rain falls, which obviates artificial watering. In long, hot, dry summers, seedlings with immature root systems have limited access to deep-stored groundwater, and transplanted seedlings commonly suffer water stress, limiting early growth or causing mortality (Close et al. 2005).

On 26 July 2009, I established an experiment in large gaps between existing vegetation on highway cut slopes at Leeming, 20 km south of Perth (32°5'6" S, 115°50'55" E). In mid-2005, topsoil had been replaced on the newly-constructed slopes, composted mulch thinly applied, various native shrubs direct seeded, and over 1,000 slender Banksias planted. Subsequently growth had been slow, with plants of mean height <75 cm providing sparse cover, and weed invasion negligible. In 2009, I planted a total of 360 slender Banksia over four 2,500 m2 replicate plots. In each replicate, I planted seedlings in 15 randomly-distributed blocks, each containing the following six treatment types: 1) Control—planted by Pottiputki hand planter (Lannen Plant Systems, Finland), with no further treatment; 2) Fertilizer tablet—planted by Pottiputki, with one 10 g Typhoon native plant tablet (Langley Fertilizers, Wangara WA) (20.7% N, 1.2% P, 10.5% K, plus trace...

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